Balanced tiltable, rotating mirror with its optical axis angularly offset from its axis of rotation

ABSTRACT

The mirror, such as a mangin-mirror, is useful in a system which automatically detects a deviation between a target and an object moving towards the target by comparing the line of sight to the target and the line of sight to the object. The mirror has a first axis about which it rotates and an optical axis which is displaced at an angle from the rotational axis. The rotational axis is kept coaxial with the target line of sight. The mirror is selectively tiltable to one of two discrete positions about its optical axis so that, as the mirror rotates, the reflection from the object forms two images, corresponding to the two mirror positions which conically scan a detector plane. Two pairs of right-angled light detectors, one for each position, are located on the plane. By comparing the time intervals for each pair of detectors when the reflection crosses the detectors, the deviation can be calculated, and by other equipment corrections in the path of the object can be made.

United States Patent [191 Chapman [4 1 May 15, 1973 [54] BALANCED TILTABLE, ROTATING MIRROR WITH ITS OPTICAL AXIS ANGULARLY OFFSET FROM ITS AXIS OF ROTATION [75] Inventor: Arthur S. Chapman, Rolling Hill Calif.

[73] Assignee: Hughes Aircraft Company, .Culver City, Calif.

[22] Filed: Mar. 13, 1970 [21] Appl. No.: 19,374

Gramm Bemis et a1. ..244/3.l6

Spangenberg ..250/236 Lozins ..250/203 R Attorney-James K. Haskell and Lewis B. Sternfels [57] ABSTRACT The mirror, such as a mangin-mirror, is useful in a system which automatically detects a deviation between a target and an object moving towards the target by comparing the line of sight to the target and the line of sight to the object. The mirror has a first axis about which it rotates and an optical axis which is displaced at an angle from the rotational axis. The rotational axis is kept coaxial with the target line of sight. The mirror is selectively tiltable to one of two discrete positions about its optical axis so that, as the mirror rotates, the reflection from the object forms two images, corresponding to the two mirror positions which conically scan a detector plane. Two pairs of right-angled light detectors, one for each position, are located on the plane. By comparing the time intervals for each pair of detectors when the reflection crosses the detectors, the deviation can be calculated, and by other equipment corrections in the path of the object can be made.

14 Claims, 4 Drawing Figures Ma's/4: 5610-! 60/4/7204 Caviar/Ma- 44/ a/zcu/r C/lCd/f PATENTEDHAY 1 5l975 Y SHEET 2 OF 2 BALANCED TILTABLE, ROTATING MIRROR WITH ITS OPTICAL AXIS ANGULARLY OFFSET FROM ITS AXIS OF ROTATION The Invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of Defense.

The present invention relates to a rotatable mirror which is tiltable about its optical axis during rotation, the optical axis being displaced at an angle from the axis of rotation. The invention is useful, for example, in systems for sensing the deviation between a particular target direction and an object moving towards the target.

In one missile guidance system, an operator guides a missile toward a moving target utilizing a sensor having a telescope with cross-hairs kept on the target. The sensor detects any angle deviation between the missile and the target direction and automatically provides a signal to alter the missiles direction of flight and to maintain the missile on target. An infrared flare at an end of the missile may be used to facilitate its detection.

The missile must be controlled during flight from its initial firing to its striking the target. At firing, it is desired, if not necessary, to have a wide angle of sight in "order to track the missile when it is close to the viewer.

To obtain such a wide angle, a wide field detector arrangement is used which, although picking up large background illumination, provides a high signal-tonoise ratio because the missile flare is still relatively close to the sensing apparatus. As the missile approaches the target, it is preferable to narrow the angle of sight and, therefore, a narrow angle detector is employed because the angle between the missile and aiming telescope is small and because the missile flare appears dimmer. The narrow angle detector increases signal-to-noise ratio.

For both detector operations, the sensor detects the position of the missile relative to the target. The center of the sensor is kept coaxial with the line of sight from the observer to the target. In particular, an image (e.g. an infrared image) from the missile is programmed into a conical scan onto a pair of orthogonal detectors, either the wide or narrow angle detectors, as described below. By measuring the time taken by the image to cross the detectors and by measuring the rotation angle, an error can be computed and error correction signals relayed to the missile to decrease the error. Such measurement and computation is fully described in Infrared System Engineering" by Richard D. Hudson, Jr., John Wiley & Sons, Inc., 1969, pp. 235-263, particularly pp 255-256. A long pair of detectors is used for wide angle sensing while a short pair of detectors is used for narrow angle sensing.

The conical scan is obtained by use of a rotating mirror having a capability of focussing all electromagnetic energy which strikes it into a single point. Among the mirrors which satisfy this requirement are mangin mirrors and parabolic mirrors. By displacing the focal point of the mirror from the center of the detector assembly, the conical scan is obtained. Displacement of the focal point is obtained by rotating the mirror about an axis which is displaced from the mirror optical axis at an angle. The mirror should be dynamically balanced to minimize vibration and to simplify bearing and support structure. For a single angular disposition of axes, the mirror can be easily balanced.

However, when two conical scans are required, as is the case in the present invention, there must be two angular dispositions of the mirror optical axis with respect to its rotational axis. This requirement is satisfied in the present invention by tilting the mirror from one discrete position to another discrete position; however, in so tilting the mirror, its moments of inertia are also changed. The present invention is directed to a mirror whose moments of inertia do not change when the mirror is tilted, that is, its three moments of inertia about the three orthogonal axes are made equal. This equality is obtained by making the mirror equivalent to a sphere of evenly distributed mass whose center is on the rotational axis and where the pivotal axis crosses the axis of rotation.

It is, therefore, an object of the present invention to provide a balanced tiltable mirror having an optical axis offset from its axis of rotation.

Other aims and objects as well as a more complete understanding of the present invention will appear from the following explanation of an exemplary embodiment and the accompanying drawings thereof, in which:

FIG. 1 is a perspective view, partly in section, of the present invention in conjunction with one illustrative use of an infrared sensor for tracking a missile towards a target;

FIG. 2 is a partial sectional view of the invention of FIG. 1;

FIG. 3 is a partial exploded perspective view of the sensor system of FIG. 1; and

FIG. 4 is a detailed perspective view of a portion of the sensor system of FIG. 1.

Accordingly, FIG. 1 illustrates a system utilizing the invention for guiding a missile 10 toward a target 12, such as a tank. An operator peers through the eyepiece 14 of a tracking telescope 16 with cross-hairs at the center of its field of view. The operator continually adjusts the orientation of the telescope to keep the crosshairs on the target 12. Secured to the telescope is a sensor apparatus 18, which includes a housing 20, so they both point in the same direction, which is the target direction. (If a mirror or prism is used to deflect light before it reaches the sensing apparatus, then the target direction is only the apparent direction in which the target is located.) Instead of fixing the tracking telescope to the sensor housing, the sensor housing 20 may be driven by a servo that is slaved to the tracking telescope. This facilitates mounting the sensor apparatus on a stabilized platform to eliminate vibrations and small disturbances.

The sensor apparatus 18 determines whether the missile 10 is in line with the target direction. If it is not, the sensor apparatus determines the angle of correction, whether up or down and/or right or left, necessary to bring the missile in line with the target. The missile may be connected to the apparatus by a wire 22 towed by the missile, or otherwise controlled by radio or other means, to deliver correction signals to the missile. Such signals cause the missile to alter its path so that it hits the target and may be effected by pivoting of the control surfaces of the missile or by firing small correction jets.

In order to enable the sensor apparatus to be able to select the missile from the background, the missile is provided with an infrared flare, as indicated by a star at indicium 24. Light from the flare passes through a window 26 and strikes a concave mirror 28. The mirror may have only a reflective coating, or may have a corrective transparent layer thereon. The mirror forms an image 24A of the flare on a detector plane 31, which is at the rear face of the window 26. A first pair of detectors 30 and 32 and a second pair of detectors 30' and 32 are located on the window at the detector plane 31 thereof. Each one of the detectors is in the form of a thin strip, and the detectors of each pair are positioned at right angles to each other. Detectors 30 and 32 are longer than detectors 30 and 32', and are respectively used for wide angle and narrow angle sensing.

The mirror 28 is mounted for rotation about an axis 34 which is caused to pass through the target, and which also passes through the point where the detectors 30 and 32 and 30 and 32' cross. However, the optical axis 36 of the mirror is displaced at an angle M from the axis of rotation. Thus, the flare is reflected from the mirror 28 as a conical scan and its image 24A is projected on the plane 31 along a circular path 38 which is concentric with the axis of rotation 34 of the mirror when the missile is on target, i.e., the missile lies on the axis 34. Under such circumstances, the flare image crosses detectors 30 and 32, or 30 and 32, depending upon which detector pair is being used, at such times and at such a rotational angle that no error correction errors are relayed to the missile. If there is no such coincidence, then the times at which the flare image crosses the detectors, when taken with the rotational angle, are detected as error from which error correction signals are forwarded to the missile.

A determination of the error, that is, a vector whose angle and direction is a measure of the deviation of the missile direction from the target direction, can be automatically made by an error computing circuit 40. One input, via lead 41, to the circuit 40 is from a mirror angle sensor 42 which indicates the precise angle of the mirror optical axis 36 from the rotational axis 34, and, therefore, the precise location of the flare image 24A on the circular path 38. Two other inputs via leads 43 and 43 to the circuit 40 are from that pair of detectors being used.

Each detector provides a brief, large, flare-indicating pulse only when the flare image 24A crosses it. If the flare indicating pulse from each detector 30 and 32, for example, coincides with the instants when the mirror optical axis crosses the detector, then the missile is on target. If the flare indicating pulse from the detector 32 occurs at the same instant as the mirror optical axis crosses that detector, but the pulse from detector 30 occurs later than the instant the optical axis crosses that detector, then the missile is directly above, or at l 2 oclock", with respect to the target. If the flare detecting output is just as late for both detectors, as is the situation shown in FIG. 1, then the missile is at 9:30 oclock" with respect to the target.

By comparing the outputs of the two detectors with the output of mirror angle sensor 42, the error computing circuit 40 can compute the direction of path correction required to bring the missile on target. The circuit 40 is connected to a missile control circuit 44 which delivers an error correction signal to the missile to direct the missile closer to the'target.

During the first period of missile flight, after the missile is launched, there generally exists a large angle between the missile and the target direction. During this period it is desirable to provide a wide detection field. However, during the later period of missile flight, when the missile is closer to the target, it is possible to utilize a smaller field of view. The present invention provides a means for effecting a change from the large field of view during the first period of missile flight to the smaller field of view with a corresponding increase in accuracy during the later period of missile flight. This change of detection fields is accomplished by pivoting the mirror 28 so that its optical axis 36 makes a greater or smaller angle with the axis of rotation 34.

During the first period of flight, a wide detector field is obtained by the use ofa wide angle M, such as 6 l2 total) between the optical axis 36 of the mirror and its axis of rotation 34. Long detectors 30 and 32 are used at this time. However, during the later period of flight, a narrow detector field is obtained by the use of a small circle 46. During this time, short detectors 30' and 32' are used. In order to enable such a change of detector field, the apparatus must be constructed to enable tilting and balanced rotation of the mirror at either of the two angles. Balanced mirror rotation is particularly important where the sensing apparatus is located on a space stabilized platform, since any vibrations caused by the mirror will be very difficult to cancel out.

As shown in FIG. 2, the mirror 28, which is a mangin type (that is, a corrective lens in front of the reflective surface) is secured to a mount 50, the mirror and mount forming a mirror assembly 52. The assembly is pivotally supported on a wheel or support 62 by pivots 64, the wheel being rotatable about axis 34. In order to achieve dynamic balance of the mirror assembly in its two tilted positions about the pivot axis defined by pivots 64, the moment of inertia of the assembly must be equal about its three perpendicular axes to make the mirror and assembly equivalent to a sphere. The center of this sphere is placed on the rotational axis of assembly 52 on the axis of pivot 64. The center, therefore, exists at the point where the rotational axis and the pivot axis cross. Furthermore, the mirror is so designed that the center of the virtual plane of the mirror is also posi tioned at the center of the sphere so that the flare image will remain in focus on the detector plane. Balancing is completed by utilizing a mass in the form of the mount and a second mass 54 so that the moment of inertia of the assembly is equal about all three axes.

The mount 50 includes a frame portion 56, which supports the mirror, an elongated rod 58 secured to the frame portion, and the mass 54 which is additionally used as an armature 60 attached to one end of the rod. The wheel 62 is rotatably mounted on the housing 20 by bearings 65 and is provided with a through bore 63 through which the rod 58 extends. A pair of faces 63a and 63b bound the ends of the wheel. A gear 66, which is fixed to the wheel at face 63b, is driven by a motor 68 to provide rotation of the wheel about the axis of rotation 34.

A stationary solenoid housing 70 with a solenoid coil 72, is located at the rear of the wheel. When the coil is not energized, the mirror assembly 52 is held in the position shown in FIG. 2 by the bias of a spring 74 which extends between the mirror assembly and the face 63a of the wheel 62. The spring biases the mirror assembly so that a portion 76 thereon abuts against a first stop or abutment 78 on the wheel face 63a. This is the position of least tilt, which is used for the narrow detection field.

When the solenoid coil 72 is energized, by applying current thereto, the mirror assembly pivots to its second position wherein a portion 80 on the frame 56 will abut against a second stop or abutment 82 on the wheel face 63a. Energization of the coil 72 magnetically urges the armature 60 toward a position where its rim 84 is best positioned to complete the magnetic flux path between an outerpole 85 and the center polepiece 86 of the solenoid. The force applied by the solenoid is greater than that exerted by spring 74 to overcome the spring force and to move the mirror assembly to the second position. The second position, which provides for a maximum tilt of the mirror optical axis 36 with the axis of rotation 34, is used for a wide detection field.

Thus, by applying current to the solenoid, the apparatus can be made to change from a narrow detection field to a wide field. The start of application of current to the solenoid can be performed manually by the operator, or automatically, after other criteria are met. The change of detector field is accomplished while the mirror is rotating, and the disturbance created during the charge lasts for a very brief period of time.

Thus the invention provides a system which automatically detects the direction of error of a missile, or other light source from a predetermined target direction. The use of a mirror to form an image at a detector plane facilitates the formation of a sharp, high intensity image, to enable very sensitive error detection under difficult conditions and for a wide range of light wave-lengths. The apparatus also enables a change from a wide to a narrow detection field in a relatively simple manner, by changing the angle of the mirror. The mirror assembly is constructed so that the mirror is well balanced at any tilt angle, using a mount that also facilitates change of tilting angle.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art.

What is claimed is:

1. Apparatus for sensing deviation of an object having a light source from a target comprising:

sensor means separated from the object and disposed at a particular detector plane;

imaging means for forming an image of the light source on the detection plane, said imaging means having a predetermined optical axis;

means for rotating said imaging means on an axis of rotation coaxial with a line extending to the target, the optical axis of said imaging means angled from the axis of rotation and crossing the axis of rotation at an intersection, to produce movement of the image generally in a circular path on the detector plane;

means responsive to said sensor means and the angular position of said imaging means about the axis of rotation, for sensing the deviation of the object from the target; and

mass means including said imaging means having a physical distribution of mass evenly distributed about the intersection of three substantially equal orthogonal moments of inertia.

2. The apparatus described in claim 1 including:

means for tilting said imaging means about the intersection during rotation thereof at a selected one of a plurality of angles different from the axis of rotation.

3. The apparatus described in claim 1 wherein:

said mass means including said imaging means comprises a mirror assembly including a mount and a mirror secured thereto;

said rotating means comprises a rotatably mounted wheel, said mirror mount being pivotally secured to said wheel for enabling pivoting of said mirror assembly about a predetermined pivot axis passing through the intersection; and

said mount having masses arranged in a manner to provide the substantially equal moments of inertia of said mirror assembly about the pivot axis, the optical axis of said mirror, and an axis mutually perpendicular to the pivot and optical axes.

4. The apparatus described in claim 1 wherein:

said said mass means including imaging means comprises a mirror assembly including a mirror and a mount, said mount including a frame portion for holding said mirror and an armature portion extending behind said mirror;

said rotating means comprises a rotatably mounted wheel, means for pivotally coupling said mirror mount to said wheel, a first stop on said wheel for limiting pivoting of said mirror assembly in a first direction, and means for biasing said mirror means towards and against said first stop; and

a solenoid disposed adjacent to said armature portion of said mount, for magnetically attracting said armature portion and to move said mirror means against said biasing means to pivot said mirror assembly to a position away from said first stop.

5. Apparatus for sensing deviation of an object having a light source from a particular target direction comprising:

a housing including a mirror mounted for pointing in the target direction;

detector means mounted in a predetermined detector plane on said housing;

means for rotating said mirror about an axis of rotation which is coaxial with the direction in which said housing is pointed;

means coupled to said mirror for tilting its optical axis at, at least, two angles from the axis of rotation, to focus light from the source substantially onto said detector plane;

means for measuring the time taken by light from said source to cross said detector means and for measuring the rotational angle of the optical axis for sensing deviation of the light source from the target direction; and

means including said mirror dynamically balancing said mirror at any of the angles of tilt.

6. The apparatus described in claim 5 wherein said sensor means comprises at least two pairs of elongated sensors; and

said mirror holding means includes means for pivoting said mirror about an axis crossing the axis of rotation to provide a plurality of angled positions of the optical axis from the axis of rotation.

7. The apparatus described in claim 5 wherein:

said means for rotating said mirror comprises a mount for rigidly holding said mirror, a wheel pivotally supporting said mount, said tilting means including means for pivoting said mount between selected positions to enable the change of angle between the axis of rotation and the optical axis, and

means for rotatably supporting said wheel on said housing.

8. A tiltable and dynamically balanced mirror assembly comprising:

a mirror rotatable about an axis of rotation and tiltable about a second axis intersecting the axis of rotation, and having an optical axis angularly offset from the axis of rotation, and

mass means including said mirror and dynamically balanced about the intersection of the axis of rotation and the second axis at all tilts of said mirror.

9. A rnirror assembly as in claim 8 further including means for tilting said mirror about the second axis, said mass means including an element cooperable with said mirror tilting means for effecting the tilting of said mirror.

10. A mirror assembly as in claim 9 wherein said tilting means includes a solenoid and said element comprises an armature of magnetic material.

11. A mirror assembly as in claim 8 further including a rotatable support having pivot means disposed along the second axis, said mass means secured to said support by said pivot means, a pair of abutments on said support and positioned on opposite sides of said pivot means, biasing means secured between a first of said abutments and said mass means for tilting said mass means and said mirror into contact with the second of said abutments, and means secured between said mass means and said support for biasing and tilting said mass means and said mirror against said first of said abutments.

12. In a missile tracking system for directing a missile towards a target comprising a missile, having an electromagnetic signal, and a sensor and error correction apparatus having a detector plane with two pairs of orthogonally positioned detector elements, one of said pair being longer than the second of said pair, a mechanism coupled to each of said detector pairs and to the missile for computing the error in missile trajectory to the target and for transmitting error correction signals to the missile for correcting the error in the missile trajectory, and a mirror assembly for directing the missile electromagnetic signal onto the detector plane and onto said pairs of detector elements, the improvement comprising:

target sighting apparatus having a target axis positionable on the target,

a stationary housing secured to said target sighting apparatus and provided with a solenoid having a center pole and end poles,

a rotatable support journalled in said housing on an axis of rotation coaxially positioned on the target axis, said support having opposed first and second faces and through bore means positioned on the axis of rotation and opening at said faces,

a drive secured to said housing and operatively coupled to said rotatable support for imparting rotation thereto,

a mount pivoted on a pivot axis to said rotatable support at said first face thereof, and having a rod extending through and beyond said bore means at said second face of said support and having an armature of ferromagnetic material secured to said rod, said armature being positioned adjacent to 6 said solenoid and movable between a first position adjacent said center pole and a second position adjacent one of said end poles,

a mirror affixed to said mount and having a curved reflective surface provided with an optical axis aimed towards the detector plane for focussing the missile electromagnetic signal onto the plane, said mirror being rotatable with said mount about the axis of rotation, and the optical axis being angularly offset to two discrete angles from the axis of rotation to provide two corresponding conical scans of the missile electromagnetic signal onto the detector plane, respectively for use with each said pair of detector elements,

a pair of stops positioned on said first face of said support on opposed sides of the pivot axis of said mount for limiting the angular movement of said mount and said mirror to said discrete angles to provide the two conical scans,

biasing means between a first of said stops and said mount for pivoting said mount into abutment with the second of said stops and for pivoting said armature in the position adjacent said solenoid center pole when said solenoid is deenergized, said solenoid when energized providing sufficient attractive force on said armature against the bias of said biasing means to pivot said mount into abutment with said first stop, and

said mount having masses, including said mirror, said rod, and said armature, so distributed as to provide a dynamic balance of said mount and said mirror at any angular position thereof.

13. Apparatus for sensing deviation of an object having a light source from a particular target direction comprising:

a housing including a mirror mounted for the target direction;

detector means mounted in a predetermined detector plane on said housing;

means for rotating and pivoting said mirror about an axis of rotation which is coaxial with the direction in which said housing is pointed, said mirror rotating and pivoting means comprising a mount rigidly holding said mirror, said mounting including a frame portion attached to said mirror and an armature portion extending behind said frame portion to provide a dynamic balance, a wheel pivotally supporting said mount, a spring extending between said wheel and said mount for urging said mount toward a first pivotal position with respect to said wheel, and a solenoid coil mounted on said wheel adjacent said armature portion of said mount for urging said mount toward a second pivotal position against the bias of said spring to enable change of angle between the axis of rotation and the optical axis, and means for rotatably supporting said wheel on said housing;

means holding said mirror so its optical axis is angled from the axis of rotation, to focus light from the source substantially onto said detector plane; and

means for measuring the time taken by light from said source to cross said detector means and for measuring the rotational angle of the optical axis for sensing deviation of the light source from the target direction.

14. A tiltable and dynamically balanced mirror assembly comprising:

a mirror rotatable about an axis of rotation and tiltable about a second axis intersecting the axis of ropointing in tation, and having an optical axis angularly offset from the axis of rotation; and

mass means secured to said mirror and dynamically balanced therewith about the intersection of the axis of rotation and the second axis for any tilt of said mirror,

ertia are substantially equal. 

1. Apparatus for sensing deviation of an object having a light source from a target comprising: sensor means separated from the object and disposed at a particular detector plane; imaging means for forming an image of the light source on the detection plane, said imaging means having a predetermined optical axis; means for rotating said imaging means on an axis of rotation coaxial with a line extending to the target, the optical axis of said imaging means angled from the axis of rotation and crossing the axis of rotation at an intersection, to produce movement of the image generally in a circular path on the detector plane; means responsive to said sensor means and the angular position of said imaging means about the axis of rotation, for sensing the deviation of the object from the target; and mass means including said imaging means having a physical distribution of mass evenly distributed about the intersection of three substantially equal orthogonal moments of inertia.
 2. The apparatus described in claim 1 including: means for tilting said imaging means about the intersection during rotation thereof at a selected one of a plurality of angles different from the axis of rotation.
 3. The apparatus described in claim 1 wherein: said mass means including said imaging means comprises a mirror assembly including a mount and a mirror secured thereto; said rotating means comprises a rotatably mounted wheel, said mirror mount being pivotally secured to said wheel for enabling pivoting of said mirror assembly about a predetermined pivot axis passing through the intersection; and said mount having masses arranged in a manner to provide the substantially equal moments of inertia of said mirror assembly about the pivot axis, the optical axis of said mirror, and an axis mutually perpendicular to the pivot and optical axes.
 4. The apparatus described in claim 1 wherein: said said mass means including imaging means comprises a miRror assembly including a mirror and a mount, said mount including a frame portion for holding said mirror and an armature portion extending behind said mirror; said rotating means comprises a rotatably mounted wheel, means for pivotally coupling said mirror mount to said wheel, a first stop on said wheel for limiting pivoting of said mirror assembly in a first direction, and means for biasing said mirror means towards and against said first stop; and a solenoid disposed adjacent to said armature portion of said mount, for magnetically attracting said armature portion and to move said mirror means against said biasing means to pivot said mirror assembly to a position away from said first stop.
 5. Apparatus for sensing deviation of an object having a light source from a particular target direction comprising: a housing including a mirror mounted for pointing in the target direction; detector means mounted in a predetermined detector plane on said housing; means for rotating said mirror about an axis of rotation which is coaxial with the direction in which said housing is pointed; means coupled to said mirror for tilting its optical axis at, at least, two angles from the axis of rotation, to focus light from the source substantially onto said detector plane; means for measuring the time taken by light from said source to cross said detector means and for measuring the rotational angle of the optical axis for sensing deviation of the light source from the target direction; and means including said mirror dynamically balancing said mirror at any of the angles of tilt.
 6. The apparatus described in claim 5 wherein said sensor means comprises at least two pairs of elongated sensors; and said mirror holding means includes means for pivoting said mirror about an axis crossing the axis of rotation to provide a plurality of angled positions of the optical axis from the axis of rotation.
 7. The apparatus described in claim 5 wherein: said means for rotating said mirror comprises a mount for rigidly holding said mirror, a wheel pivotally supporting said mount, said tilting means including means for pivoting said mount between selected positions to enable the change of angle between the axis of rotation and the optical axis, and means for rotatably supporting said wheel on said housing.
 8. A tiltable and dynamically balanced mirror assembly comprising: a mirror rotatable about an axis of rotation and tiltable about a second axis intersecting the axis of rotation, and having an optical axis angularly offset from the axis of rotation, and mass means including said mirror and dynamically balanced about the intersection of the axis of rotation and the second axis at all tilts of said mirror.
 9. A mirror assembly as in claim 8 further including means for tilting said mirror about the second axis, said mass means including an element cooperable with said mirror tilting means for effecting the tilting of said mirror.
 10. A mirror assembly as in claim 9 wherein said tilting means includes a solenoid and said element comprises an armature of magnetic material.
 11. A mirror assembly as in claim 8 further including a rotatable support having pivot means disposed along the second axis, said mass means secured to said support by said pivot means, a pair of abutments on said support and positioned on opposite sides of said pivot means, biasing means secured between a first of said abutments and said mass means for tilting said mass means and said mirror into contact with the second of said abutments, and means secured between said mass means and said support for biasing and tilting said mass means and said mirror against said first of said abutments.
 12. In a missile tracking system for directing a missile towards a target comprising a missile, having an electromagnetic signal, and a sensor and error correction apparatus having a detector plane with two pairs of orthogonally positioned detector elements, onE of said pair being longer than the second of said pair, a mechanism coupled to each of said detector pairs and to the missile for computing the error in missile trajectory to the target and for transmitting error correction signals to the missile for correcting the error in the missile trajectory, and a mirror assembly for directing the missile electromagnetic signal onto the detector plane and onto said pairs of detector elements, the improvement comprising: target sighting apparatus having a target axis positionable on the target, a stationary housing secured to said target sighting apparatus and provided with a solenoid having a center pole and end poles, a rotatable support journalled in said housing on an axis of rotation coaxially positioned on the target axis, said support having opposed first and second faces and through bore means positioned on the axis of rotation and opening at said faces, a drive secured to said housing and operatively coupled to said rotatable support for imparting rotation thereto, a mount pivoted on a pivot axis to said rotatable support at said first face thereof, and having a rod extending through and beyond said bore means at said second face of said support and having an armature of ferromagnetic material secured to said rod, said armature being positioned adjacent to said solenoid and movable between a first position adjacent said center pole and a second position adjacent one of said end poles, a mirror affixed to said mount and having a curved reflective surface provided with an optical axis aimed towards the detector plane for focussing the missile electromagnetic signal onto the plane, said mirror being rotatable with said mount about the axis of rotation, and the optical axis being angularly offset to two discrete angles from the axis of rotation to provide two corresponding conical scans of the missile electromagnetic signal onto the detector plane, respectively for use with each said pair of detector elements, a pair of stops positioned on said first face of said support on opposed sides of the pivot axis of said mount for limiting the angular movement of said mount and said mirror to said discrete angles to provide the two conical scans, biasing means between a first of said stops and said mount for pivoting said mount into abutment with the second of said stops and for pivoting said armature in the position adjacent said solenoid center pole when said solenoid is deenergized, said solenoid when energized providing sufficient attractive force on said armature against the bias of said biasing means to pivot said mount into abutment with said first stop, and said mount having masses, including said mirror, said rod, and said armature, so distributed as to provide a dynamic balance of said mount and said mirror at any angular position thereof.
 13. Apparatus for sensing deviation of an object having a light source from a particular target direction comprising: a housing including a mirror mounted for pointing in the target direction; detector means mounted in a predetermined detector plane on said housing; means for rotating and pivoting said mirror about an axis of rotation which is coaxial with the direction in which said housing is pointed, said mirror rotating and pivoting means comprising a mount rigidly holding said mirror, said mounting including a frame portion attached to said mirror and an armature portion extending behind said frame portion to provide a dynamic balance, a wheel pivotally supporting said mount, a spring extending between said wheel and said mount for urging said mount toward a first pivotal position with respect to said wheel, and a solenoid coil mounted on said wheel adjacent said armature portion of said mount for urging said mount toward a second pivotal position against the bias of said spring to enable change of angle between the axis of rotation and the optical axis, and means for rotatably supporting said wheel on said housing; means holdiNg said mirror so its optical axis is angled from the axis of rotation, to focus light from the source substantially onto said detector plane; and means for measuring the time taken by light from said source to cross said detector means and for measuring the rotational angle of the optical axis for sensing deviation of the light source from the target direction.
 14. A tiltable and dynamically balanced mirror assembly comprising: a mirror rotatable about an axis of rotation and tiltable about a second axis intersecting the axis of rotation, and having an optical axis angularly offset from the axis of rotation; and mass means secured to said mirror and dynamically balanced therewith about the intersection of the axis of rotation and the second axis for any tilt of said mirror, said mass means, when taken with said mirror, being physically distributed about the intersection to form the equivalent of a sphere of evenly distributed mass whose three orthogonal moments of inertia are substantially equal. 